Typical analog displays, such as in vehicle instrument panels,utilized air core gauges to position pointers in relation to sensor values. The basic air core gauge mechanism includes a substantially circular disk of magnetized material that is fixed to a spindle and surrounded by at least two coils of wire, with at least one coil typically perpendicular to another of the coils. When electric current passes through the coils, a magnetic field is produced exerting a force on the magnetized disk. The angular direction of the field produced by the coils primarily depends on the number of ampere-turns in each of the coils. The resultant magnetic field produced can be represented by the vector addition of the fields produced by each of the coils.
The basic two coil air core gauge is typically driven by one of two methods. In the first method, shown in FIG. 1, the two coils are designated by reference numerals 36 and 38. Coil 38 is biased to a fixed voltage V.sub.ign, through load resistor 34. The voltage across coil 36 varies with a signal from variable resistance sensor 32 comprising, with load resistor 30, a voltage divider between V.sub.ign and ground. As the voltage across coil 36 varies, the direction of the resultant magnetic field, with which the rotor rotates to align itself, varies.
In the second basic method, shown in FIG. 2, a signal on line 50 from a sensor (not shown), typically a signal with a frequency varying with a vehicle parameter, is converted to an analog signal through circuit 52 and input into a sine,cosine drive circuit 56. The sine/cosine drive circuit 56 generates a signal on line 58 proportional to the cosine of the desired angle of deflection of the rotor and a signal on line 60 proportional to the sine of the desired angle of deflection of the rotor. Coils 62 and 64, in response to the signals on lines 58 and 60, develop magnetic fields with sine and cosine component magnetic vectors correlating to the desired pointer rotation. Various other methods which are not set forth here are also used to drive air core gauges.
Conventional drive methods for air core gauges, including the methods set forth above, are typically open loop systems in which actuation currents are applied to the coils without the use of any feedback information as to the actual pointer position to correct the values of the currents.
Pointer direction errors can arise from many sources. Some of these sources include stray magnetic fields, such as from a nearby radio speaker magnet. To prevent the affect of stray magnetic fields on the gauge, a magnetic shunting enclosure (can) is typically mounted around the coils and rotor of the gauge. The shunting cans must be made from expensive alloys or heat treated to keep from becoming magnetized by the coils, which would also cause errors.
Other pointer direction errors can arise from variances in the diameter of the wire used to wind the coils. As the diameter of the wire changes, the impedance changes, which may cause the current through the wire to change. Pointer direction errors can occur because of unbalanced or heavy pointers. Also, the damping fluid typically used in the air core gauges can cause a mechanical hysteresis which may lead to pointer errors.
Reduction of pointer direction errors is desirable in automotive systems because gauge errors can lead to limits on gauge design, warranty costs for gauges with high inaccuracies, and increased calibration cost to prevent the errors.